1. Field of the Invention
The present invention relates to an intrusion detecting apparatus for a vehicle, and more particularly to an intrusion detecting apparatus for a vehicle which is suitable to detect an illegal intrusion of a person by utilizing an ultrasonic doppler shift signal which is generated when a person intrudes into the passenger compartment of a vehicle.
2. Description of the Related Art
Conventionally, an intrusion detecting apparatus for a vehicle of this type is known as disclosed in, for example, Japanese Patent Publication (Kokoku) No. 7-5062.
In this publication, when an ultrasonic transmitter provided in a passenger compartment of the vehicle receives an output pulse signal from a pulse oscillator and transmits an ultrasonic signal into the passenger compartment of the vehicle, an ultrasonic receiver provided in the passenger compartment of the vehicle receives the ultrasonic signal which has been reflected by a person or the like in the passenger compartment of the vehicle. Then the received ultrasonic signal is mixed with the output pulse signal from the pulse oscillator, and the mixed signal is filtered by a low pass filter so that a doppler signal is output.
Then, the envelope of the doppler signal is detected by a detector. When the detected envelope signal is higher than a reference voltage, a comparator outputs a signal which is then integrated by an integrating circuit. Then, when the integrated output from the integrating circuit, that is an integration level of the compared signal with respect to time, reaches a predetermined level, it is detected that a person has illegally intruded.
By this construction, in order to detect an illegal intrusion of a person based on a doppler signal, the detecting circuit, the comparator and the integrating circuit are employed as mentioned above. Therefore, as long as the comparator outputs a signal as mentioned above, the integrating circuit integrates the compared signal even when the doppler signal is generated due to any other disturbance.
In other words, as long as the comparator outputs a compared signal due to, for example, air turbulence in the passenger compartment of the vehicle because a door of the passenger compartment of the vehicle is slightly open, the integrating circuit continues to integrate the compared signal. The air turbulence, i.e., disturbance, may be generated by air fluctuations in the passenger compartment or by movements of material bodies such as a figurine stuck on a window glass. Therefore, even when the integration level reaches the predetermined level, it may erroneously be detected that a person has illegally intruded.
Because of this, the user of this vehicle may make the intrusion detecting apparatus inoperative or may ignore an alarm generated by the erroneous detection.